Caste developmental system of the Japanese damp-wood termite Hodotermopsis japonica (Isoptera: Termopsidae)

Caste developmental pathways of the Japanese damp-wood termite Hodotermopsis japonica Holmgren were estimated by measuring body parts. From the size distribution of head-width, seven peaks were recognized in populations of larvae. Of these peaks, the seventh has a wide range and variance, indicating that it represents several instars. The individuals that make up the seventh peak, which take the role of workers (pseudergate), are suggested to enter the nymphal stage(s), which has brachypterous wing buds, prior to becoming alates. The pseudergates appear to be totipotent, that is, capable of differentiating into any caste, including reproductives and soldiers. Sixth instars and pseudergates have the potential to become soldiers via a presoldier stage. Although the evolution of caste systems in termites is controversial, the linear developmental pathway found in this study is thought to be the typical pattern in lower termites. Sexual discrimination was also investigated, males and females being easily distinguished by examining the seventh abdominal sternites of individuals older than fourth instar larvae. Both sexes appear to follow the same caste developmental pathway.     

Histological analysis of fat body development and molting events during soldier differentiation in the damp-wood termite, Hodotermopsis sjostedti (Isoptera, Termopsidae)

The caste system of termites is well defined, with a high degree of polyphenism among colony members. Polyphenic caste characteristics are hormonally regulated, and juvenile hormone (JH) is particularly involved in caste determination, as is the case with many other social insects. In the present study, soldier differentiation in the damp-wood termite, Hodotermopsis sjostedti, was induced by treatment with a JH analog (pyriproxyfen) in order to establish the chronology of tissular modifications appearing in response to the hormone.The fat body is involved in the physiological events that prepare the insect for the molting transition. The development of the fat body started within three days after hormonal treatment, and it filled the entire abdominal cavity for about four days prior to the molt to presoldier, maintaining this state until the next molt to soldier. Fat body development was accompanied by the accumulation of protein granules in the cytoplasm, but these granules disappeared during the few days preceding the molt to presoldier. The timing of consumption of these storage proteins corresponded to the window of epidermal growth, which was conspicuous about 14 days after hormonal treatment, and synthesis of the new cuticle, which was initiated 10 days after treatment. We summarize the chronology of the histological events under hormonal control.     

Juvenile hormone titers and caste differentiation in the damp-wood termite Hodotermopsis sjostedti (Isoptera, Termopsidae)

Termites are social insects, presenting morphologically distinct castes, performing specific tasks in the colony. The developmental processes underlying caste differentiation are mainly controlled by juvenile hormone (JH). Although many fragmentary data support this fact, there was no comparative work on JH titers during the caste differentiation processes. In this study, JH titer variation was investigated using a liquid chromatography-mass spectrometry (LC-MS) quantification method in all castes of the Japanese damp-wood termite Hodotermopsis sjostedti, especially focusing on the soldier caste differentiation pathway, which was induced by treatment with a JH analog. Hemolymph JH titers fluctuated between 20 and 720pg/microl. A peak of JH was observed during molting events for the pseudergate stationary molt and presoldier differentiation, but this peak was absent prior to the imaginal molt. Soldier caste differentiation was generally associated with high JH titers and nymph to alate differentiation with low JH titers. However, JH titer rose in females during alate maturation, probably in relation to vitellogenesis. In comparison, JH titer was surprisingly low in neotenics. On the basis of these results in both natural and artificial conditions, the current model for JH action on termite caste differentiation is discussed and re-appraised.     

Soldier-like intercastes in the rotten-wood termite Hodotermopsis sjostedti (Isoptera: Termopsidae)

All termite species possess a distinct sterile-soldier caste in their colonies, although reproductive soldiers, with soldier characteristics and reproductive ability, have been reported from several species of the family Termopsidae. Such intercastes have been considered the primitive-soldier caste, and based on this many researchers have discussed the evolutionary origin of termite soldiers. We investigated whether such soldier-reproductive intercastes also exist in the Japanese rotten-wood termite Hodotermopsis sjostedti. Abnormal individuals with soldier-like characteristics were found and designated as soldier-like intercastes, which appeared to have both soldier and reproductive characteristics. Based on our morphometric analyses and histological examinations, we suggest that the developmental origin of this intercaste is a pseudergate, nymph or sixth-instar larva. In addition, the intercaste was found to have relatively well-developed gonads, although mature oocytes and spermatozoa were not found in female ovaries and male seminal vesicles, respectively. We conclude that the soldier-like intercaste of H. sjostedti does not have reproductive ability, which is different from all other known soldier-like intercastes in Termopsidae.     

Histology of the hormone-producing glands in the damp-wood termite <Emphasis Type="Italic">Hodotermopsis sjostedti</Emphasis> (Isoptera,Termopsidae): A focus on soldier differentiation

Caste differentiation in termites depends on complex hormonal changes during postembryonic development. Juvenile hormone (JH) is a central player in this process. The present study examined histological changes in the main hormone-producing endocrine glands, the corpora allata and molt glands, in the Japanese dampwood termite Hodotermopsis sjostedti. We focused on the soldier caste differentiation pathway, which can be induced artificially using an analogue of JH. The corpora allata exhibited volumetric changes during soldier induction, reflecting variations in the quantity of cytoplasm. Corpora allata from alates and neotenics clearly showed differentiation accompanied by cell proliferation, preparing for the high-level JH production necessary for reproduction. However, the volume increase of corpora allata was not always correlated to high JH titers. In contrast, molt glands degenerated in the reproductive castes. The JH analogue induced hypertrophy of the molt glands, along with the formation of lacunae, possibly related to ecdysteroid production. The JH analogue effect, inducing soldier differentiation, was suggested to require both mimic of high JH titers and stimulation of the molt glands. Received 12 November 2007; revised 2 June 2008; accepted 14 July 2008.     

Regulation of neotenic differentiation through direct physical contact in the damp-wood termite Hodotermopsis sjostedti

Insectes Sociaux - In cooperative societies such as those of ants, honey bees, and termites, the number of reproductives is often regulated by social interactions. In many termite species, helper...     

Screening of genes expressed in developing mandibles during soldier differentiation in the termite Hodotermopsis sjostedti

We investigated the morphological changes accompanying soldier differentiation in the damp-wood termite Hodotermopsis sjostedti. Genes expressed in the developing mandibles, which undergo the most remarkable morphological changes during soldier differentiation, were screened using fluorescent differential display. Database searches for sequence similarities were conducted and the relative expression levels were then quantified by real-time polymerase chain reaction. Among the identified candidate genes, 12 genes were upregulated during soldier differentiation. These included genes for cuticle proteins, nucleic acid binding proteins, ribosomal proteins and actin-binding protein, which were inferred to be involved in caste-specific morphogenesis in termites.     

Systematics of rove beetles (Coleoptera: Staphylinidae: Aleocharinae) associated with Hodotermopsis sjostedti (Isoptera: Termopsidae)

Members of the termitophilous subtribe Termitozyrina (Aleocharinae: Lomechusini) associated with Hodotermopsis sjostedti are taxonomically treated. The genera Hodotermophilus Naomi & Terayama and Termophidoholus Naomi & Hirono, and each type species (monotypic) are redescribed. Termophidoholus formosanus, originally described from Taiwan, and its host termite H. sjostedti are newly recorded from Laos. Yakuus iwatai Kanao & Maruyama gen. & sp. nov. and H. gloriosus were collected sympatrically in the same nests in Yaku‐shima, Japan. All of the above species, belonging to the three genera, share the presence of a batch of spurs at the tibial apex of fore and mid legs. Habitus photographs and illustrations of diagnostic features are provided for these species, and their phylogenetic relationships are discussed based on morphological similarities and the extant distribution of the host termite species.     

Cryoprotection in dampwood termites (Termopsidae, Isoptera)

In contrast to the majority of the Order, the dampwood termites of the family Termopsidae found in colder regions can experience frost and snow, either in cool temperate areas at high latitudes (45°), or alpine areas at high elevations (>1000 m). This suggests that dampwood termites are adapted to cold climates. We investigated this hypothesis in two dampwood termites, Porotermes adamsoni Froggatt and Stolotermes victoriensis Hill. We measured nest temperatures and atmospheric temperatures of their alpine habitat during winter, and measured survival and recovery at subzero temperatures. We also determined the minimum temperature at which these species remain active and the LT50 values. We used a novel gas chromatographic strategy to examine eight metabolites from individuals of both species collected in winter and summer to identify possible cryoprotectants. Both P. adamsoni and S. victoriensis had significantly higher levels of trehalose, a known cryoprotectant, in winter than in summer; in addition S. victoriensis also had higher levels of unsaturated fatty acid ligands in winter than in summer, consistent with patterns observed for cold adaptation in other organisms. These results are the first to reveal that dampwood termites are adapted to cold climates and use trehalose and unsaturated lipids as cryoprotectants.     

Compound eye development during caste differentiation in the termite Reticulitermes speratus (Isoptera: Rhinotermitidae)

We morphologically examined postembryonic compound eye development in Reticulitermes speratus (Kolbe) to understand developmental regulation during caste differentiation. The eye primordia were shown to exist from the larval stage. The number of ommatidia and compound eye size greatly increased over the course of imaginal development. Nymphoids (second-form reproductives) possessed a developed compound eye structure on the surface of the cuticle and thick optic nerves, but individual ommatidia were not clearly discriminated. However, in the line of apterous workers and soldiers, although the outer rim of the eye was observed from second-stage workers, there were few morphological differences among instars, including ergatoids (third-form reproductives). Both nymphoids and ergatoids are slightly physogastric and have highly developed reproductive organs. These results suggest that eye development in the apterous line could be strongly arrested and that there is a weak developmental correlation between the eyes and reproductive organs in R. speratus.     

Knockdown of the juvenile hormone receptor gene inhibits soldier-specific morphogenesis in the damp-wood termite Zootermopsis nevadensis (Isoptera: Archotermopsidae)

The Methoprene-tolerant (Met) protein has been established as a juvenile hormone (JH) receptor. Knockdown of the Met gene caused precocious metamorphosis and suppression of ovarian development. However, the function of Met in caste development of social insects is unclear. In termites, JH acts as a central factor for caste development, especially for soldier differentiation, which involves two molts from workers via a presoldier stage. Increased JH titer in workers is needed for the presoldier molt, and the high JH titer is maintained throughout the presoldier period. Although presoldiers have the fundamental morphological features of soldiers, the nature of the cuticle is completely different from that of soldiers. We expected that JH signals via Met are involved in soldier-specific morphogenesis of the head and mandibles during soldier differentiation, especially in the presoldier period, in natural conditions. To test this hypothesis, we focused on soldier differentiation in an incipient colony of the damp-wood termite Zootermopsis nevadensis. Met homolog (ZnMet) expression in heads increased just after the presoldier molt. This high expression was reduced by ZnMet double stranded (dsRNA) injection before the presoldier molt. Although this treatment did not cause any morphological changes in presoldiers, it caused strong effects on soldiers, their mandibles being significantly shorter and head capsules smaller than those of control soldiers. Injection of ZnMet dsRNA throughout the presoldier stage did not affect the formation of soldier morphology, including cuticle formation. These results suggested that the rapid increase in ZnMet expression and subsequent activation of JH signaling just after the presoldier molt are needed for the formation of soldier-specific weapons. Therefore, besides its established role in insect metamorphosis, the JH receptor signaling also underlies soldier development in termites.     

Caste differentiation pathways in the Neotropical termite Silvestritermes euamignathus (Isoptera: Termitidae)

Termites are hemimetabolous insects that commonly exhibit a bifurcation in post‐embryonic development, with the presence of two lines: (i) the nymphal–imaginal line; and (ii) the apterous line. The former comprises the reproductive imagoes of the colonies and their preceding immature stages, whereas the latter is composed of workers, soldiers and their precursors. In this study, we analyzed the developmental pathways of caste differentiation in the Neotropical termite Silvestritermes euamignathus and compared it to the known sibling species, Silvestritermes holmgreni. Our results show that the apterous line is composed of male and female workers without sexual dimorphism and that soldiers are exclusively females. The imaginal line comprises five instars of nymphs, which may develop into either nymphoid neotenics or alates. Similarities found in our study between caste patterns of S. euamignathus and of S. holmgreni suggest a shared evolutionary history in these termite species. Some biological differences between the species are also discussed.     

Identification of soldier caste-specific protein in the frontal gland of nasute termite Nasutitermes takasagoensis (Isoptera: Termitidae)

The termite soldier is unique because of its defensive task in a colony. In Nasutitermitinae (family Termitidae), soldiers use in their defense frontal glands, which contain various chemical substances. To isolate the gene products related to the chemical defense, we compared the sodium dodecyl sulfate-polyacrylamide gel electrophoresis protein profiles of soldier heads with those of workers of the nasute termite Nasutitermes takasagoensis. We identified a 26-kDa soldier-specific protein (Ntsp1) that exists most abundantly in the dorsal head including the frontal gland. We determined the N-terminal amino acid sequence of Ntsp1, and then cloned the Ntsp1 cDNA by rapid amplification of the cDNA ends-polymerase chain reaction (RACE-PCR). A putative signal peptide was detected upstream of the N-terminus and the Ntsp1 protein showed sequence homologies with known insect secretory carrier proteins, which bind to hydrophobic ligands such as juvenile hormone, suggesting that Ntsp1 belongs to this class of proteins. Northern blot analysis confirmed that the expression level of Ntsp1 was high only in the soldier head. In addition, the localization of Ntsp1 expression was limited in epithelial cells of the frontal gland reservoir, suggesting that this protein binds to some terpenoid(s) preserved in the frontal gland reservoir.     

Patterns of neotenic differentiation in a subterranean termite, Reticulitermes speratus (Isoptera: Rhinotermitidae)

Plasticity in the caste developmental pathway is a remarkable characteristic of termite societies. In Reticulitermes, two types of neotenic reproductive, nymphoids and ergatoids, may differentiate from nymphs and workers and take over reproduction in the colony after the death of the original primary reproductive pair. We examined the dynamics of newly differentiated nymphoids and ergatoids in experimentally orphaned laboratory colonies of R. speratus with different caste compositions. The period required for differentiation of nymphoids was shorter than that for differentiation of ergatoids. The sex ratio of neotenics was strongly female‐biased, particularly in ergatoids. The results suggested that the number of differentiated ergatoids was restricted by the existence of nymphs or nymphoids in a colony. Workers were assumed to kill most newly differentiated neotenics. Attack reflecting conflict between colony members is probably an important mechanism to control neotenic emergence.     

The labrum of Schedorhinotermes minor soldier (Isoptera,Rhinotermitidae)

The labrum of Schedorhiontermes minor soldier was examined to determine the distribution, types and structures of sense organs present with both light microscopy and scanning and transmission electron microscopy. Three types of sensilla were recognized: companiform sensillae and two types of sensillae trichodea, the short bristles and the long bristles. The companiform sensillae are singly-innervated receptors distributed in large numbers on the anterolateral margins of the labrum. The short and long bristles are multi-innervated; the former are found all over the surface, the latter are much more scarce and are found only at the front of the labrum. Both are at the same time chemo- and mechanoreceptors. The labrum is strewn with many glandular pits and exhibits at its apex a brush of cuticular not innervated spines. On the basis of ultrastructural evidence and of what we know of the defense mechanisms of the soldier, a functional interpretation is attempted.     

Mate preference and disease risk in Zootermopsis angusticollis (Isoptera: Termopsidae)

Termites face significant and chronic intranidal selection pressures from parasites and pathogens that colonize their nests. They also encounter microbes outside their nest while foraging and during dispersal of winged primary reproductives to establish new colonies. The latter run the additional risk of becoming infected by a mating partner. Indeed, death of reproductives because of disease is a major cause of incipient colony failure and may favor prescreening prospective mates for signs of illness. To determine the role of disease on mate preference in termites, female primary reproductives of the Pacific dampwood termite Zootermopsis angusticollis (Hagen) simultaneously were presented with reproductive males that were either healthy or exhibiting a progression of symptoms associated with infection by the entomopathogenic fungus Metarhizium anisopliae (Metchnikoff Sorokin). We compared duration and frequency of female visits to healthy and infected males. In addition, we determined the physiological consequences for females exposed to fungal conidia, either directly or indirectly through their mate. Females showed no preference for healthy rather than infected males. Moreover, only directly-exposed females experienced negative physiological effects, having a reduced chance of survival, gaining less weight, developing fewer functional ovarioles, and producing significantly fewer vitellogenic oocytes than controls. Although there are important fitness-related costs of direct exposure, the lack of mate selection based on disease risk suggests that more imminent ecological pressures (e.g., predators, desiccation) override the need for a careful and time-consuming assessment of a potential mate's health.